A compact engine is employed as a power source in an electric generator and a portable work tool such as a grass-trimmer, a blower, a chain-saw, and a power cutter. FIG. 9 shows a conventional grass-trimmer 300 as a typical example of the work tool. The grass-trimmer 300 has a right end portion provided with a main body 310 in which an engine is accommodated, a left end portion provided with a cutting blade 320, and an intermediate portion provided with a handle 330. Rotation of the engine is transmitted to the cutting blade 320, so that the rotating cutting blade 320 performs cutting to a plant, or trimming to a hedge while an operator grips the handle 330.
An air cooled engine is widely used for the work tool because of the need for a compact and light-weight engine. In the air cooled engine, a cooling fan is fixed to a drive shaft of the engine, and the fan is continuously rotated during engine operation for generating a cooling air to forcibly cool an engine component such as a cylinder. The cylinder of the air-cooled engine is generally cylindrical and has an inner side in which a combustion chamber is defined, and has an outer side provided with a plurality of heat radiation fins for enhancing cooling efficiency to the cylinder. Heat dissipation can be efficiently performed when the cooling air passes through a space between the neighboring fins, thereby efficiently performing cooling to the cylinder.
In order to secure safety for the operator, the cooling fan and the cylinder which is operated at a high temperature are covered with a cover made of a resin. A cover portion covering the cylinder (cylinder cover) is configured to allow the cooling air generated at the cooling fan to flow through the cooling fins for efficient cooling to the cylinder. However, size and shape of the cylinder cover is subjected to restriction so as to install the engine in its entirety to the work tool.
On the other hand, melting may occur in the cylinder cover having a low heat resistivity if an inner surface of the cylinder cover is in contact with the cylinder or fins. Further, the cylinder and the cylinder cover involve dimensional tolerance and positioning tolerance. When a narrow gap between the cylinder and the cylinder cover is contemplated, various problems may occur from the practical perspective such as: the cylinder and the cylinder cover are in continuous contact with each other due to dimensional error; or these may be contacted with each other due to engine vibration; or intensive vibration occurs in the cylinder cover even though contact between the cylinder and the cylinder cover is avoided. Taking the above in mind, a gap between the cylinder and the cylinder cover should be set as narrow as possible yet allowing cooling air to flow through the gap but avoiding such above-described drawbacks.
However, high cooling efficiency may not be attainable in case that cooling air flows through such narrow gap. To overcome this problem, Japanese Patent No. 3726065 discloses an air guide plate provided between the cylinder cover and the cylinder. The air guide plate can efficiently flow cooling air particularly along the cylinder region, thereby providing high cooling efficiency. The air guide plate extends in a direction parallel to the inner surface of the cylinder cover.